1. Field of the Invention
The present invention relates to a device for controlling positioning of a moving article and, more particularly, to a device with a position detector for detecting the position of a moving article and for determining the position of the moving article in accordance with the detected position. The present invention may be used for a machine tool and an industrial machine, and the moving article may be a workpiece operated upon by the machine tool.
2. Description of the Prior Art
It is very important to accurately sense a position of a moving article, for example, a tool of a machine tool, since the approach speed of the tool should be controlled in accordance with the relationship between a workpiece and the tool. The position of the tool is detected by a position detector, for example, a rotary encoder or a linear encoder. The output of the position detector is connected to a controller in which the position of the moving article is determined on the basis of the output of the position detector and which outputs a speed control signal to a control circuit. The control circuit comprises a drive circuit for a driving means for moving the moving article in accordance with the speed control signal from the control circuit. A positioning control device is composed by the position detector, the controller and the control circuit.
In such a positioning control device, abnormal conditions of the position detector or signal lines connected thereto cause the machine tool to malfunction. Thus, the positioning control device provides an abnormal condition detecting means which detects the abnormal conditions of the position detector and the like so that the controller feeds an abnormality judging signal to the control circuit to stop the moving article. The positioning control device mentioned above requires a signal line in addition to position signals 4 and 5 for detecting the abnormal conditions of the position detector and the like.
One method is performed such that an abnormality signal is generated by the position detector itself independently of a position signal, whereby an abnormal condition of the position detector is judged based on the abnormality signal. The other method is such that the controller monitors position signals from the position detector at all times, whereby a good or bad positional signal is judged based on the data of movement of an article to be controlled, and emits a signal of judgement.
There have been presented the disadvantages that, out of the above-described methods, the former method of detecting an abnormality needs an abnormality signal in addition to a position signal, while the latter method of detecting an abnormality requires that the data of movement of an article to be controlled must be transmitted to the controller by some means.